We observed the changes of the gastrocnemius muscle in relation to the percentage of lengthening of the rabbit's tibia by callotasis. 75 rabbits were separated into 3 lengthening groups, 10, 20, and 30 percent lengthening, respectively. Histopathologic observations, based on the fiber size variation, intemalization of the nuclei, degeneration, regeneration, and endo-mysial fibrosis of muscle fibers, revealed that substantial changes occurred in the latter groups. Histo-morphometrically, the decrease in the mean size of Types I and II muscle fibers was observed in all lengthening groups, but there was no significant change in the proportion of the muscle fiber types in any of the lengthening groups.